Graphene is a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice. It can be wrapped up into 0D fullerenes, rolled into 1D nanotubes or stacked into 3D graphite. Graphenes are attracting renewed interest recently due to the advances in micromechanical exfoliation and epitaxial growth methods that make macroscopic two-dimensional sheets of sp2-hybridised carbon atoms available for fundamental research. Sheets comprising a single layer and a few layers of graphene have been grown epitaxially by chemical vapour deposition of hydrocarbons on metal substrates for example Pt (Land, et al, STM investigation of single layer graphite structures produced on Pt(111) by hydrocarbon decomposition. Surf. Sci., 264, 261-270 (1992)) and TiC (Itoh et al, Scanning tunneling microscopy of monolayer graphite epitaxially grown on a TiC(111) surface. Surf. Sci. Lett., 254, L437-L442 (1991)), or by thermal decomposition of SIC (Bommel, A. J. V. et al., Leed and auger electron observations of the SiC(0001) surface. Surf. Sci., 48 463-472 (1975); Berger, C. et al. Ultrathin epitaxial graphite: 2D electron gas properties and a route toward graphene-based nanoelectronics. J. Phys. Chem. B, 108 19912-19916 (2004); Berger, C. et al. Electronic confinement and coherence in pattern epitaxial graphene. Science, 312, 1191-1196 (2006)). However, these methods are substrate specific and therefore cannot be formed on substrates of general interest to semiconductor electronics. They also require a high temperature post-anneal step to develop the graphene structure.
There have also been attempts to grow graphene by adopting the same approach as that used in growing carbon nanotubes involving, for example, the pyrolysis of hydrocarbons (heavy oil) in a carbon arc (Krishnan, A. et al. Graphitic cones and nucleation of curved carbon surfaces. Nature, 388, 451-454 (1997); Dujardin, E., et al., Fabrication of mesoscopic devices from graphite microdisks. Appl. Phys. Lett., 79, 2474-2476 (2001)). In order to improve the crystallinity of the resulting microstructures, a post anneal step of typically above 2000° C. is required. This approach produces graphite stacks containing typically 60 to 100 layers of graphene sheets. This approach produces thick graphite aggregates, but not single or few-layer sheets of interest for electronic or mechanical applications. Further, all of the approaches mentioned above require a high temperature process step (>1000° C.) which limits their application in producing organic electronic devices.
Attempts have also been made to isolate graphene sheets from graphite by first intercalating with an oxidising acid and reintercalating with alkali metal under heating, for hours, followed by reaction with ethanol. However, this results in partially exfoliated structures of thick aggregated stacks of graphene sheets (Viculis, et al., Intercalation and exfoliation routes to graphite nanoplatelets. J. Mater. Chem., 15, 974-978 (2005)) that are not so suitable for subsequent processing or for electronic applications.
Novoselov et al (Novoselov, K. S. et al. Electrical field effect in atomically thin carbon films. Science, 306, 666-669 (2004)) obtained graphene sheets containing a single and a few layers of graphene by mechanical exfoliation (repeat peeling) of small mesas of highly oriented pyrolytic graphite (HOPG) and transferring these onto a desired substrate. This approach, however, presents considerable challenges for up-scaling to large substrate areas, or for producing bulk quantities of graphenes.
It would be advantageous to be able to make solution-processable or solution-dispersible graphene sheets. It would also be advantageous if said graphene sheets could be readily deposited onto desired substrates and/or electrode structures at room temperature or at slightly elevated temperatures.
Graphite oxide (GO) is a potential precursor to graphene upon thermal de-oxidation or chemical reduction. Although GO itself has been studied for over a century, its structure and properties remain elusive, and significant progress towards dispersability, the first step for applications, has been made only recently. It would certainly be very desirable to be able to solution-process these materials, for example by printing; thus opening opportunities for electronics on large and/or flexible substrates that take advantage of the 2D nature of these materials.
Graphite can be converted into graphite oxide in an aqueous medium (for example, see Hummers et al, Preparation of graphitic oxide. J. Am. Chem. Soc., 80, 1339 (1958) and Schniepp, H. C. et al., Functionalised single graphene sheets derived from splitting graphite oxide, J. Phys. Chem. B, 110, 8535-8539 (2006)). The prerequisites that enable the preparation of so called bulk graphene are complete oxidation of graphite and extreme rapid heating of the resulting GO. The complete oxidation of graphite produces stoichiometric GO. This well-known process adds oxygen-based chemical groups to the graphite surface, for example, selected from epoxide, hydroxyl and carboxylic acid groups, and results in the bulk graphite being completely separated into single sheets. However, these single sheets are at most partially stable and only in alkaline solution in the presence of a large excess of ions (e.g. 0.01 to 1M sodium hydroxide) which is undesirable for most applications. It would therefore be desirable to have dispersions comprising single sheets or few-layer sheets that are not required to be stabilised in an alkaline medium with a high concentration of ions because the presence of excess ions can interfere with the electronic properties of the graphene sheets.
Niyogi, S. et al in Solution properties of graphite and graphene. J. Am. Chem. Soc., 128 7720-7721 (2006) describe the attempted functionalisation of fully oxidised graphite oxide with octadecylamine at 120° C. and attempted redispersion in organic solvents. However, the achievable concentration of dispersed functionalised graphene sheets was low.
Stankovich et al. in Synthesis and exfoliation of isocyanate-treated graphene oxide nanoplatelets, Carbon, 44 3342-3347 (2006), and Graphene-based composite materials. Nature, 442 (2006), 282-286, describe the functionalisation of fully oxidised GO sheets with organic isocyanate groups in DMF and reported a dispersability of up to 1 mg/mL. However, the sheets are not readily dispersible in non-polar solvents. This method itself depends on the presence of a sufficiently high density of hydroxyl groups and carboxylic acid groups to react with the isocyanate to form carbamides. According to the work of the present inventors this method is not suitable for GO which is not fully oxidised due to the much lower density of hydroxyl groups.
Strong surface-surface attraction between graphene sheets prevents them from forming stable dispersions in solvent systems and 3D graphite reforms from the stacking of the graphene sheets. Therefore, a challenge to find a process that yields a uniform or substantially uniform distribution of single graphene sheets remains.
In view of the above, there remains a need to find a way to make stable, highly concentrated dispersions comprising graphene containing sheets.
The present invention seeks to address at least some of the above challenges by controlling the degree of oxidation of graphite when forming graphite oxide and, optionally controlling the degree to which the surface of the graphite oxide is functionalised, and after such functionalisation optionally reducing the oxidation state of the graphene oxide to a desired value. Further it is an aim of this invention to: (i) make sub-stoichiometric GO materials; (ii) functionalise said sub-stoichiometric GO materials to render them more water soluble or more soluble in organic solvents by, for example, surface-grafting the GO with, for example, suitable solubilising groups; (iii) chemically reduce the functionalised materials for further applications.